As a grinding machine for grinding the circumferential surface or end surface of a workpiece having a circular column shape, conic trapezoidal shape, or cylindrical shape, or in other words, having an outer circumferential surface with a circular cross-sectional shape, such as a roller (cylindrical roller or conical roller) or bearing rings of a rolling bearing, a so-called centerless grinding machine that performs grinding of a workpiece while holding the outer-circumferential surface of the workpiece using rollers or shoe is known, referring to JPS62148147 (A) and JP2009136953 (A). FIG. 9 and FIG. 10 illustrate an example of conventional construction of a 2 roller 1 shoe type of such a centerless grinding machine.
The grinding machine 1 comprises an upper roller 2 and a lower roller 3 that are arranged on both the top and bottom sides of a circular column shaped workpiece W. In a state that the center axes of the upper roller 2 and the lower roller 3 are arranged in the horizontal direction and in parallel to each other, the upper roller 2 and the lower roller 3 rotate at the same speed in the same direction so that the workpiece W is rotated in a state of being supported from both the top and bottom sides. Moreover, in order to simplify the supply and output of a workpiece W, the upper roller 2 is supported so as to be able to rock in the vertical direction about a rocking center (not illustrated in the figure), and presses the workpiece W that is arranged below the upper roller 2 toward the lower roller 3 using the elastic force of a spring (not illustrated in the figure). On the other hand, the lower roller 3 is supported so as to be able to rotate only around the center axis of the lower roller 3, and is not able to move. A shoe 4 is arranged between the upper roller 2 and the lower roller 3 in the vertical direction so as to extend in a direction that is orthogonal to the center axes of the upper roller 2 and lower roller 3. The shoe 4 is such that the tip-end surface thereof comes in contact with the outer-circumferential surface of the workpiece W, and supports the workpiece W from the side. Moreover, a backing plate 5 is arranged on one side in the axial direction of the workpiece W, and one end surface in the axial direction of the workpiece W butts against the backing plate 5. The workpiece W is rotated around the center axis that is arranged in the horizontal direction while being held in the processing position by the upper roller 2, lower roller 3, shoe 4 and backing plate 5. In this state, the outer-circumferential surface or end surface of the workpiece W is ground by a rotating grindstone 6 that is arranged on the other end side in the axial direction of the workpiece W.
In the case of a grinding machine 1 having conventional construction, due to the low degree of freedom of movement of the upper roller 2 and lower roller 3, it is necessary to prepare plural upper rollers 2 and lower rollers 3 having different sizes and shapes according to the type of workpiece W, specifically, kind and size of each component of a bearing.
FIG. 11A to FIG. 11C illustrate the state of performing grinding of three kinds of workpieces W1, W2, W3 using a conventional grinding machine. Workpiece W1 has a large-diameter circular column shape, workpiece W2 has a circular column shape with a smaller diameter than workpiece W1, and workpiece W3 has a conic trapezoidal shape, the inclination of the generating line thereof being different than that of workpiece W1. In the grinding machine 1, the upper roller 2 is capable of only rocking displacement in the vertical direction, and the lower roller 3 is fastened so that movement is not possible, so the upper roller 2A and lower roller 3A illustrated in FIG. 11A that are suitable for the grinding of workpiece W1 and that have a small diameter, the outer diameter of which does not change along the axial direction, cannot be used for processing workpiece W2 and workpiece W3; the upper roller 2B and lower roller 3B illustrated in FIG. 11B that are suitable for the grinding of workpiece W2 and that have a large diameter, the outer diameter of which does not change along the axial direction, cannot be used for processing workpiece W1 and workpiece W3; and the upper roller 2C and lower roller 3C that are suitable for the grinding of workpiece W3, the outer diameter of which changes along the axial direction (the outer-circumferential surface is a partial conical convex surface), cannot be used for processing workpiece W1 and workpiece W2. In this way, in the grinding machine 1, every time the outer diameter and the inclination state of the generating line differ, it is necessary to prepare an upper roller 2 (2A to 2C) and a lower roller 3 (3A to 3C) that are suitable for the respective grinding. Therefore, particularly on a production line for producing various kinds of small rods, it is not possible to avoid an increase in the amount of labor and time for replacing the rollers. Management of the roller diameter (outer diameter) and performance such as wear resistance are strictly required for the upper roller 2 and lower roller 3, and are very expensive. Therefore, having to prepare special rollers for each workpiece, also poses a problem from the aspect of manufacturing cost.
Moreover, in the conventional grinding machine 1, a workpiece W that is placed underneath the upper roller 2 is pressed toward the lower roller 3 by the elastic force of a spring. The pressure force by the upper roller 2 is constant according to the elastic force of the spring (physical characteristics such as the length and wire diameter of the spring), so in order to change the pressure force by the upper roller 2, it is necessary to replace the spring with a different spring. In JPS62148147 (A), a grinding machine is disclosed in which it is possible to appropriately change the pressure force by the upper roller. However, in this grinding machine, a special and complex mechanism is required just for changing the pressure force, so it is not possible to avoid an increase in size and manufacturing cost of the grinding machine.